Nowadays, semiconductor light-emitting devices, such as light-emitting diodes, have been used for a wide variety of applications, e.g., illumination and remote control. To ensure high functional reliability and a low power requirement of the semiconductor light-emitting devices, as great as possible, the external quantum efficiency is required from the devices.
In principle, the external quantum efficiency of a semiconductor light-emitting device is determined both by the internal quantum efficiency and extraction efficiency. The internal quantum efficiency is determined by the material property and quality. The extraction efficiency means the proportion of radiation emitted from the interior of the device into surrounding air or encapsulating epoxy. The extraction efficiency is determined by the losses occurring when radiation leaves the interior of the device. One of the main causes for such losses is the radiation proportion resulting from the high optical refraction coefficient of the semiconductor material, e.g., about 3.6 for gallium arsenide (GaAs), that cannot be emitted at the semiconductor surface on account of total reflection. In the case of GaAs, a critical angle for the total reflection of 16.2.degree. results at the transition to air. By the direct path, only the proportion of radiation that hits the boundary surface below a lower angle to the surface normal is emitted. However, this radiation proportion is still subject to a partial reflection caused by the abrupt change in the refraction coefficient. For radiation hitting the boundary surface vertically, the transmission coefficient is about 68%, so that if the absorption of the radiation on the way to the boundary surface is ignored, only about 2.7% of the generated radiation can leave the semiconductor crystal on a direct path in the case of a flat structure.
Some prior arts regarding the fabrication of semiconductor light-emitting devices disclose the way for enhancing the external quantum efficiency of the semiconductor light-emitting devices by using surface roughening method, and are listed as follows: U.S. Pat. Nos. 5,898,192; 5,429,954; and 5,040,044.
However, the foregoing prior arts still indicate the need for a new surface roughening method applicable to any kind of semiconductor light-emitting devices. Specially for a top-most layer formed of (100)-oriented GaP, it usually serves as a window layer of a semiconductor light-emitting device having an active layer of AlGaInP, and its surface is still difficult to be roughened uniformly so far. Therefore, it is desired that the new surface roughening method can be performed to a top-most layer (covering layer) of a semiconductor light-emitting device, which is formed of a customary semiconductor material such as GaP, GaAsP and AlGaAs. Moreover, the top-most layer can be roughened by using the method regardless of lattice orientation of the semiconductor material made into the top-most layer. It is also desirable that a surface-roughened top-most layer of a semiconductor light-emitting device, made by use of the surface roughening method, has an uniform surface roughness. It is also desirable that the surface roughening method is a reproducible and low-cost procedure. The present invention is directed toward satisfying the aforesaid need.